Kraft paper is a strong, tough, break-resistant paper manufactured from pulp using the kraft process, and used primarily in heavy packaging applications, as a material for making cardboards, and for envelopes, adhesive tapes, etc. In heavy packaging applications, kraft paper is processed into sacks and filled with several tens of kilograms of cement, rice, flour, and various other products, for example, for storage or transport. Accordingly, kraft paper must be strong enough not to break in sack forms and JIS-P3401 specifies Kraft Paper Types 1 to 5, each covering different applications, etc., and meeting the standardized tensile strength, tear strength, and other characteristics of certain levels or greater.
On the other hand, Clupak refers to equipment that inserts a paper web between a roll and an endless rubber blanket to compress the paper web with a nip bar and the rubber blanket, while at the same time the pre-stretched blanket shrinks to cause the paper web to also shrink and thereby increase its breaking elongation, and this equipment is used to provide increased breaking elongation to kraft paper used in heavy packaging applications as mentioned above.
As for how kraft paper is manufactured using this Clupak (hereinafter referred to as “Clupak paper”), Patent Literature 1 describes kraft paper made with Clupak, whose weight per area is in a range of 73 g/m2 or more but less than 84 g/m2, which meets the standard values under JIS P3412, and whose air permeance as specified in JIS P8117 is 4 to 10 seconds.
Patent Literature 2 describes kraft paper used as a sack-shaped decorative or reinforcement kraft paper characterized in that it comprises a single layer of 95 to 130 g/m2 in basis weight and is creped by a Clupak system to achieve a product of lateral tensile strength and lateral breaking elongation (the latter is measured in compliance with JIS-P8113) of 30 to 65, which is characterized by using material pulp adjusted to have a freeness of between 450 and 650 cc.